It's usually preferable to place microphones close to the speaker when recording, where great differences in sound can be achieved with subtle changes in position and direction.
A good starting place is to point the mic directly at the speaker, somewhere between the centre and the edge of the cone. Generally, the closer you place the mic to the centre of the cone, the brighter and harsher the sound will be, whereas going closer to the edge will give a softer and warmer tone.
If a cardioid mic is placed very close to the cone you will experience the 'proximity effect', giving the sound increased bass response, and although this effect is usually avoided in most recording situations, it can work very well with guitar amps, giving a really beefy sound. Varying the angle of the mic and distance from the speaker will give subtly different results, so it's worth setting aside plenty of time to experiment.
It's important that you always reference your monitors in isolation from the live sound - even if the sound coming from the guitar amp is great, you have to make sure it translates well to the recording. If you don't have the luxury of a separate recording space and an assistant engineer, try wearing closed back headphones when setting up the mic - this allows accurate monitoring of the recording without the distraction of the direct sound from the amplifier.
The use of additional microphones positioned further away from the amp will pick up more ambient sound and generally complement the close mic setup. When using this method, always be aware of phase cancellation; this is where two very similar signals are received a short time apart (due to the differences in distance from the source) causing potential problems on playback. If the signal is played back on a mono device it can disappear altogether, so a good way to test for potential problems is by pressing your desk or DAW's mono button. You can correct any problems caused by phase cancellation on most larger mixing desks and DAWs by simply inverting the phase of one of the signals. Even if cancellation is not a big problem, it's worth playing with the phase of the different mic signals as this can yield radically different results.